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Sea level changes revealed by cosmic dusts: A pilot study of the Upper Permian reefs in Guizhou, China
Journal of Geochemical Exploration 88 (2006) 431 – 435 www.elsevier.com/locate/jgeoexp
Sea level changes revealed by cosmic dusts: A pilot study of the Upper Permian reefs in Guizhou, China Yongchao Lu a,*, Xinong Xie a, Tao Jiang a, Ping Chen a, Yaoqi Zhou b a
b
Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China Institute of Petroleum Resources and Environmental Geology, China University of Petroleum, Dongying, Shandong, 257062, China Received 1 April 2005; accepted 19 August 2005 Available online 8 November 2005
Abstract In this study, we test the hypothesis that the distribution of Co, characteristic of cosmic dusts, in platform carbonates may trace palaeo-environment changes. 23 samples collected from the Upper Permian reefs in the carbonate platform margin at Ziyun, Guizhou, have been measured by neutron activation analysis. The variation of Co in sedimentary rocks is used to calculate the variations in sedimentary rate and high-frequency sea level changes. The results are in good agreement with the integrated interpretation of sequence stratigraphy. Thus, the enrichment of Co from cosmic dusts may be useful for the analysis of highresolution sequence stratigraphy. D 2005 Elsevier B.V. All rights reserved. Keywords: Cosmic dust; Co; Sea level changes
1. Introduction Alvarez et al. (1980) were the first to present a method to calculate sedimentary rate based on Ir enrichment, an element characteristic of cosmic dust. The content of Ir in cosmic dusts is less than 10 9 ppm but is much less than that of Co with a mean value of 4000 10 6 ppm (Alvarez et al., 1980). Zhou et al. (1998) suggested that the contents of Ir and Co in cosmic dust show a positive linear correlation based on the analysis of 23 samples from the P–T boundary in the South China. In this paper, variations of Co content are invoked to calculate the sedimentary rate of platform carbonates. Like Ir, the concentration of Co in cosmic dust is much higher
* Corresponding author. Fax: +86 27 67883051. E-mail address: [email protected] (Y. Lu). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.099
than the values recorded in the continental crust (i.e., less than 25 10 6 ppm) (Wu and Zhou, 2000). If the meteoric flux is constant with time, the slower the sedimentary rate, the larger the Co content will be. If the annual sedimentary rate for cosmic dusts is assumed to be a constant, the richness of Co in sedimentary strata can be used to calculate variations of sedimentary rate and water depth for the strata. In this study, Co is used to document variations in the sedimentary rate in the Upper Permian reefs in the Guizhou Province, China. 2. Materials and methods In this study, samples are collected from Upper Permian reefs in the Ziyun County of the southern Guizhou province, where numerous, well exposed, Late Permian reefs are located at a continental marginal taphrogenic trough, in southern part of the
432
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
upper Yangtze platform. The Ziyun reefs are located at the break of the platform and trough and forms an arc-shape break line along the platform margin with Longlin–Ceheng–Wangmo–Luodian in southeastern Guizhou and northwestern Guangxi (Fig. 1). According to geological investigation and outcrop sequence stratigraphy analysis (Luo et al., 1990), the Upper Permian exposed in the Ziyun area mainly consists of the Wujiapingian and Changxingian and can be divided into five sequences (Fig. 2). Sequence V in the Ziyun section is predominantly formed by the uppermost Permian reef complex units and shows the accretionary characteristics of platform marginal barrier reefs. The Ziyun reef complex is considered to be a third-order sequence of the Upper Permian, which developed under the maximum transgressive setting during the last phases of the Upper Permian. Three system tracts have been recognized in the sequence, i.e., the lowstand system tract (LST) dominated by an incised valley and a slope fan, the transgressive system tract (TST) consists of lower TST (LTST) and upper TST (UTST), and highstand system tract (HST). Each system tract has a different architecture of genetic facies and frame-building organisms and a special parasequence stacking pattern, indicating the development and growth strate-
gies of the reef accretion under different eustatic conditions. 23 samples are collected from the Changxingian sequence (Sq5) in the Ziyun section, Guizhou. In order to eliminate contaminations and purify Co, some chemical treatments have been used during the sampling procedure, such as sulphur and carbamide deposition, chelated hydronium exchanging resin, cation exchanging resin and organic solvent extraction. Co is measured by neutron activation. 3. Calculation of sedimentary rate and water depth If the cosmic dust input is assumed to be constant with time, the sedimentary rate can be presented as follows: VS ¼ VS VAVðCoÞ=½ AðCoÞ kAWðCoÞ
ð1Þ
k ¼ AðLaÞ=AWðLaÞ ¼ AðSmÞ=AWðSmÞ
ð2Þ
where V SV and AV(Co) are the average fluxes of cosmic dust and of Co; AW(Co) is the Co background of the sample; k is the contribution of sediments source to the sample and can be calculated by formula (2) based on the constant distribution of lanthanum of the earth’s surface rocks. If Co partly comes
Fig. 1. Generalized location map and cross section of the geotectonic background of Ziyun area, Southeastern Guizhou.
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
433
Fig. 2. The stratigraphic lithofacies of Shitouzhai reef-core section, Southeastern Guizhou.
Fig. 3. Co content and sedimentary rate (Sr) along the Changxingian Fm at Ziyun (see Fig. 1 for the legend of the stratigraphic column).
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Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
from the land, it increases with continental discharge. Hence, Co in marine sediments originates from two sources: (1) meteoric dust and (2) sediments transported by rivers. Consequently, formula (1) should be changed as follows: VS ¼VS Vf½ AVðCoÞ kVAWðCoÞg=½ AðCoÞkAWðCoÞ
ð3Þ
where kV is the contribution of Co transport from land in the standard sample. The relationship between the water depth and transport distance in a marine environment can be calculated following the formula of Stockes: v ¼ k2 gr2 ðd1 d2 Þu1 :
ð4Þ
Thus, water depth (h) can be calculated by the following formula: h ¼ dtanh ¼ k1 tanh=r3
ð5Þ
where v is the vertical sedimentary rate; g is the gravity constant; d 1 and d 2 are the density of sediments and water, respectively; u is a constant of the medium; h is the slope of sea floor; r is grain size, and k 1, k 2 are Stockes constants. v s, the sedimentary rate of compacted rocks, is linked to the original sedimentary rate v: v ¼ k3 vs
ð6Þ
Combined with formula (4), we get r2 ¼ k3 vs = k2 g ðd1 d2 Þu1
ð7Þ
Then, combining formulae (7) and (5), we get 3=2 h ¼ k1 tanh= k3 vs = k2 gðd1 d2 Þu1 :
ð8Þ
For a specific marine environment, h, u, d 1, d 2, k 1, k 2, k 3 and g are the constants; thus, the formula can be simplified as follows: 3=2
H ¼ k0 =VS
ð9Þ
where k0 ¼ k1 tanh k2 g ðd1 d2 Þu1 =k3 g3=2 :
ð10Þ
Therefore, sedimentary rate and relative water depth of depositional units can be calculated by formulae (3) and (9).
4. Results Compositions of the 23 samples from the Ziyun reef complex, southeastern Guizhou, are given in Fig. 3. The Co content is listed in the Table 1. According to formulae (3) and (9), the calculated sedimentation rate and relative water depth are shown in Fig. 3. The major results can be summarized as follows: (1) Th Co content is closely related to the lithology of carbonate rocks. It is higher in the bafflestone reef facies than in the capping bed facies or reef canal facies. (2) There are high contents of Co near the erosional surface, such as in the samples of ZD11, ZC50 and ZC47. (3) The relative eustatic curve given by the chemical sequence stratigraphic method reflects very clearly the structure of parasequences. It shows that reef canal facies sediments were formed by wave erosion during HST and filled with retrogradation formed in the TST. At the top of reef facies, there is usually laminated limestone deposited in the HST period. 5. Conclusion Although our results have demonstrated that the recovery of the sedimentary rate and relative water depth (h) can be estimated by the enrichment of Co in sediments, we recognize that there are still many problems that need to be resolved, such as the effect, on the enrichment of Co, of extraterrestrial events and loss due to aerial erosion. A better constraint of this techTable 1 The content of Co in the sequence V of Changxingian, South China Samples
ZD14 ZD13 ZD11 ZD10 ZD9 ZD7 ZD6 ZD5 ZD3 ZC111 ZC100 ZC92
Co
Samples
Values ( 10 2 ppm)
Error (%)
0.45 0.502 0.705 1.3 0.188 0.587 0.691 0.26 0.252 0.342 0.372 0.306
11.7 13 8.9 6 19.8 8.9 8.4 18 16 13.7 13.5 15
ZC86 ZC74 ZC64 ZC58 ZC53 ZC50 ZC49 ZC48 ZC47 ZC46 ZC45
Co Values ( 10 2 ppm)
Error (%)
1.13 0.7 0.419 0.776 1.06 0.151 0.041 0.264 0.14 1.694 1.041
7.2 8.3 12.3 6.9 7.5 35 0 28 49 5.7 9
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
nique could be obtained through the study of oceanic cores. References Alvarez, L.W., Alvarez, W., Asaro, F., Michel, 1980. Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science 208, 1095 – 1107.
435
Luo, Z.L., Jin, Y.Z., Zhao, X.K., 1990. The emei taphrogenesis in the Upper Yangtze platform. Geological Magazine 127 (5), 393 – 405. Wu, Zhiping, Zhou, Yaoqi, 2000. Using the characteristics elements from meteoritic must in strata to calculate sedimentation rate. Acta Sedimentologica Sinica 18 (3), 395 – 399. Zhou, Yaoqi, Wu, Zhiping, Shi, Puqing, 1998. Applications of neutron activation analysis in sequence stratigraphy. Earth Science Frontiers 5 (1–2), 143 – 149.
Sea level changes revealed by cosmic dusts: A pilot study of the Upper Permian reefs in Guizhou, China Yongchao Lu a,*, Xinong Xie a, Tao Jiang a, Ping Chen a, Yaoqi Zhou b a
b
Faculty of Earth Resources, China University of Geosciences, Wuhan, 430074, China Institute of Petroleum Resources and Environmental Geology, China University of Petroleum, Dongying, Shandong, 257062, China Received 1 April 2005; accepted 19 August 2005 Available online 8 November 2005
Abstract In this study, we test the hypothesis that the distribution of Co, characteristic of cosmic dusts, in platform carbonates may trace palaeo-environment changes. 23 samples collected from the Upper Permian reefs in the carbonate platform margin at Ziyun, Guizhou, have been measured by neutron activation analysis. The variation of Co in sedimentary rocks is used to calculate the variations in sedimentary rate and high-frequency sea level changes. The results are in good agreement with the integrated interpretation of sequence stratigraphy. Thus, the enrichment of Co from cosmic dusts may be useful for the analysis of highresolution sequence stratigraphy. D 2005 Elsevier B.V. All rights reserved. Keywords: Cosmic dust; Co; Sea level changes
1. Introduction Alvarez et al. (1980) were the first to present a method to calculate sedimentary rate based on Ir enrichment, an element characteristic of cosmic dust. The content of Ir in cosmic dusts is less than 10 9 ppm but is much less than that of Co with a mean value of 4000 10 6 ppm (Alvarez et al., 1980). Zhou et al. (1998) suggested that the contents of Ir and Co in cosmic dust show a positive linear correlation based on the analysis of 23 samples from the P–T boundary in the South China. In this paper, variations of Co content are invoked to calculate the sedimentary rate of platform carbonates. Like Ir, the concentration of Co in cosmic dust is much higher
* Corresponding author. Fax: +86 27 67883051. E-mail address: [email protected] (Y. Lu). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.099
than the values recorded in the continental crust (i.e., less than 25 10 6 ppm) (Wu and Zhou, 2000). If the meteoric flux is constant with time, the slower the sedimentary rate, the larger the Co content will be. If the annual sedimentary rate for cosmic dusts is assumed to be a constant, the richness of Co in sedimentary strata can be used to calculate variations of sedimentary rate and water depth for the strata. In this study, Co is used to document variations in the sedimentary rate in the Upper Permian reefs in the Guizhou Province, China. 2. Materials and methods In this study, samples are collected from Upper Permian reefs in the Ziyun County of the southern Guizhou province, where numerous, well exposed, Late Permian reefs are located at a continental marginal taphrogenic trough, in southern part of the
432
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
upper Yangtze platform. The Ziyun reefs are located at the break of the platform and trough and forms an arc-shape break line along the platform margin with Longlin–Ceheng–Wangmo–Luodian in southeastern Guizhou and northwestern Guangxi (Fig. 1). According to geological investigation and outcrop sequence stratigraphy analysis (Luo et al., 1990), the Upper Permian exposed in the Ziyun area mainly consists of the Wujiapingian and Changxingian and can be divided into five sequences (Fig. 2). Sequence V in the Ziyun section is predominantly formed by the uppermost Permian reef complex units and shows the accretionary characteristics of platform marginal barrier reefs. The Ziyun reef complex is considered to be a third-order sequence of the Upper Permian, which developed under the maximum transgressive setting during the last phases of the Upper Permian. Three system tracts have been recognized in the sequence, i.e., the lowstand system tract (LST) dominated by an incised valley and a slope fan, the transgressive system tract (TST) consists of lower TST (LTST) and upper TST (UTST), and highstand system tract (HST). Each system tract has a different architecture of genetic facies and frame-building organisms and a special parasequence stacking pattern, indicating the development and growth strate-
gies of the reef accretion under different eustatic conditions. 23 samples are collected from the Changxingian sequence (Sq5) in the Ziyun section, Guizhou. In order to eliminate contaminations and purify Co, some chemical treatments have been used during the sampling procedure, such as sulphur and carbamide deposition, chelated hydronium exchanging resin, cation exchanging resin and organic solvent extraction. Co is measured by neutron activation. 3. Calculation of sedimentary rate and water depth If the cosmic dust input is assumed to be constant with time, the sedimentary rate can be presented as follows: VS ¼ VS VAVðCoÞ=½ AðCoÞ kAWðCoÞ
ð1Þ
k ¼ AðLaÞ=AWðLaÞ ¼ AðSmÞ=AWðSmÞ
ð2Þ
where V SV and AV(Co) are the average fluxes of cosmic dust and of Co; AW(Co) is the Co background of the sample; k is the contribution of sediments source to the sample and can be calculated by formula (2) based on the constant distribution of lanthanum of the earth’s surface rocks. If Co partly comes
Fig. 1. Generalized location map and cross section of the geotectonic background of Ziyun area, Southeastern Guizhou.
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
433
Fig. 2. The stratigraphic lithofacies of Shitouzhai reef-core section, Southeastern Guizhou.
Fig. 3. Co content and sedimentary rate (Sr) along the Changxingian Fm at Ziyun (see Fig. 1 for the legend of the stratigraphic column).
434
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
from the land, it increases with continental discharge. Hence, Co in marine sediments originates from two sources: (1) meteoric dust and (2) sediments transported by rivers. Consequently, formula (1) should be changed as follows: VS ¼VS Vf½ AVðCoÞ kVAWðCoÞg=½ AðCoÞkAWðCoÞ
ð3Þ
where kV is the contribution of Co transport from land in the standard sample. The relationship between the water depth and transport distance in a marine environment can be calculated following the formula of Stockes: v ¼ k2 gr2 ðd1 d2 Þu1 :
ð4Þ
Thus, water depth (h) can be calculated by the following formula: h ¼ dtanh ¼ k1 tanh=r3
ð5Þ
where v is the vertical sedimentary rate; g is the gravity constant; d 1 and d 2 are the density of sediments and water, respectively; u is a constant of the medium; h is the slope of sea floor; r is grain size, and k 1, k 2 are Stockes constants. v s, the sedimentary rate of compacted rocks, is linked to the original sedimentary rate v: v ¼ k3 vs
ð6Þ
Combined with formula (4), we get r2 ¼ k3 vs = k2 g ðd1 d2 Þu1
ð7Þ
Then, combining formulae (7) and (5), we get 3=2 h ¼ k1 tanh= k3 vs = k2 gðd1 d2 Þu1 :
ð8Þ
For a specific marine environment, h, u, d 1, d 2, k 1, k 2, k 3 and g are the constants; thus, the formula can be simplified as follows: 3=2
H ¼ k0 =VS
ð9Þ
where k0 ¼ k1 tanh k2 g ðd1 d2 Þu1 =k3 g3=2 :
ð10Þ
Therefore, sedimentary rate and relative water depth of depositional units can be calculated by formulae (3) and (9).
4. Results Compositions of the 23 samples from the Ziyun reef complex, southeastern Guizhou, are given in Fig. 3. The Co content is listed in the Table 1. According to formulae (3) and (9), the calculated sedimentation rate and relative water depth are shown in Fig. 3. The major results can be summarized as follows: (1) Th Co content is closely related to the lithology of carbonate rocks. It is higher in the bafflestone reef facies than in the capping bed facies or reef canal facies. (2) There are high contents of Co near the erosional surface, such as in the samples of ZD11, ZC50 and ZC47. (3) The relative eustatic curve given by the chemical sequence stratigraphic method reflects very clearly the structure of parasequences. It shows that reef canal facies sediments were formed by wave erosion during HST and filled with retrogradation formed in the TST. At the top of reef facies, there is usually laminated limestone deposited in the HST period. 5. Conclusion Although our results have demonstrated that the recovery of the sedimentary rate and relative water depth (h) can be estimated by the enrichment of Co in sediments, we recognize that there are still many problems that need to be resolved, such as the effect, on the enrichment of Co, of extraterrestrial events and loss due to aerial erosion. A better constraint of this techTable 1 The content of Co in the sequence V of Changxingian, South China Samples
ZD14 ZD13 ZD11 ZD10 ZD9 ZD7 ZD6 ZD5 ZD3 ZC111 ZC100 ZC92
Co
Samples
Values ( 10 2 ppm)
Error (%)
0.45 0.502 0.705 1.3 0.188 0.587 0.691 0.26 0.252 0.342 0.372 0.306
11.7 13 8.9 6 19.8 8.9 8.4 18 16 13.7 13.5 15
ZC86 ZC74 ZC64 ZC58 ZC53 ZC50 ZC49 ZC48 ZC47 ZC46 ZC45
Co Values ( 10 2 ppm)
Error (%)
1.13 0.7 0.419 0.776 1.06 0.151 0.041 0.264 0.14 1.694 1.041
7.2 8.3 12.3 6.9 7.5 35 0 28 49 5.7 9
Y. Lu et al. / Journal of Geochemical Exploration 88 (2006) 431–435
nique could be obtained through the study of oceanic cores. References Alvarez, L.W., Alvarez, W., Asaro, F., Michel, 1980. Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science 208, 1095 – 1107.
435
Luo, Z.L., Jin, Y.Z., Zhao, X.K., 1990. The emei taphrogenesis in the Upper Yangtze platform. Geological Magazine 127 (5), 393 – 405. Wu, Zhiping, Zhou, Yaoqi, 2000. Using the characteristics elements from meteoritic must in strata to calculate sedimentation rate. Acta Sedimentologica Sinica 18 (3), 395 – 399. Zhou, Yaoqi, Wu, Zhiping, Shi, Puqing, 1998. Applications of neutron activation analysis in sequence stratigraphy. Earth Science Frontiers 5 (1–2), 143 – 149.